As a method for manufacturing chloropolysilane represented by Formula 1 having n being an integer of 2 or more, Patent Literature 1 discloses that a mixed product having high selectivity of hexachlorodisilane is obtained by reacting silicon alloy or metallic silicon with chlorine using an oscillating reactor. It further discloses that hexachlorodisilane is obtained by the reaction at a relatively low temperature in the range of 120° C. to 250° C. when a silicon alloy such as ferrosilicon, calcium silicon, or magnesium silicon is used. It also discloses that a higher reaction temperature in the range of 300° C. to 500° C. is preferable when metallic silicon is used as a raw material but the yield of hexachlorodisilane is lowered if the temperature exceeds 500° C.
Patent Literature 2 describes a method for manufacturing tetrachlorosilane and discloses that tetrachlorosilane can be obtained by reacting chlorine diluted to three to ten times (in volume ratio) by an inert gas with metallic silicon in a semifluid state at a reaction temperature in the range of 450° C. to 800° C., although a method for manufacturing chloropolysilane is not described. In this case, it describes that the purity of the metallic silicon is preferably 90% or more because reaction residues can be reduced such as chlorides of Ti, Fe, Al, etc. It further describes that: the reaction is extremely slow when the reaction temperature is lower than 450° C.; a reaction temperature in the range of 600° C. to 800° C. is preferable; and the upper limit of the reaction temperature is limited to 800° C. because the corrosion of a reactor becomes a problem at such a high temperature.
Patent Literature 3 discloses a method of improving the production yield of chloropolysilane by adding copper or a copper compound of preferably 0.1% to 20% by weight to silicon particles and conducting a chlorination reaction. In this case, it describes that the silicon particles as metallic silicon desirably have high purity because the quantity of a solid by-product caused by impurities is small and that preferable purity is 97% or more. It discloses that a preferable temperature of the chlorination reaction is 140° C. to 300° C. and that the production yield of chloropolysilane is lowered if the temperature exceeds 300° C.
In this way, as conventional technologies on a method of obtaining chloropolysilane by chlorinating silicon alloy or metallic silicon, it has been known that a chlorination reaction can be conducted at a relatively low temperature when a silicon alloy of a low silicon content such as ferrosilicon or calcium silicon is used as a raw material. Since it has been known, however, that the reaction is extremely slow at a temperature lower than 450° C. in the reaction of obtaining silicon tetrachloride by reacting metallic silicon with chlorine, it is assumed that a higher temperature tends to be required for chlorination reaction as the purity of the metallic silicon increases. Although the temperature of the reaction for obtaining chloropolysilane can be lowered by using metallic silicon including impurity metals such as iron and calcium having a catalytic activity, the problem has been that chlorinated products of iron and calcium derived from impurity metals are produced and solidified as by-products, and it has been an industrial challenge.
In response, Patent Literature 3 discloses that it is possible to conduct a chlorination reaction at a relatively low temperature in the range of 140° C. to 300° C. by adding copper or a copper compound to silicon even when high-purity silicon particles preferably having purity of 97% or more are used as the raw material to obtain chloropolysilane. The reason why silicon particles of high purity are preferable is that the quantity of solid by-products derived from impurities is small. However, Patent Literature 3 does not describe an object to obtain high-purity chloropolysilane. It does not describe a specific value of the purity of silicon raw material or of the product chloropolysilane. That is, a solution to the problem in obtaining high-purity chloropolysilane is not clearly specified in Patent Literature 3. Further, in consideration of industrial continuous reaction, copper or a copper compound accumulates unavoidably in a reactor when a raw material essentially including copper or a copper compound is added in the reactor, the problem in the solidification of by-products rises in the same manner as iron and calcium, and hence there are still problems for industrial application.
Patent Literature 3 describes that, when obtained chloropolysilane is used as a material of semiconductor silicon or amorphous silicon, the chloropolysilane is used after it is once reduced to the form of SinH2n+2. Thus, since contamination may possibly occur again at a succeeding reduction process even when purification is applied in the state of chloropolysilane, refinement and purification is conducted after the final product in the form of SinH2n+2 is obtained in the commonly used procedure. Consequently, at the time Patent Literature 3 was applied, it was not necessary to increase the purity of chloropolysilane so much, and the problem of manufacturing high-purity chloropolysilane itself was not recognized.
In recent years, however, it has been confirmed that, when hexachlorodisilane is used directly as a silicon source for amorphous silicon semiconductor, the growth rate of a silicon film in chemical vapor deposition (CVD) is very large and the electrical properties of the formed film are excellent. Thereafter, a method of directly using hexachlorodisilane as a material for CVD has been used drastically. Moreover, hexachlorodisilane is used also in atomic layer deposition (ALD) allowing formation of a uniform film of one atomic layer level, hence the hexachlorodisilane itself is required to have high purity of a ppm level. Thus, a new problem of how to obtain high-purity hexachlorodisilane used as a semiconductor material is arising. Further, the application of chloride of a higher order such as octachlorotrisilane is also studied.
The present inventors have applied distillation purification to obtain high-purity hexachlorodisilane, and found that chlorides derived from Al and Ti, which are impurities included in metallic silicon, are hardly separable from chloropolysilane because the chloride of Al has a sublimating property and the chloride of Ti has a boiling point close to hexachlorodisilane. The inventors assumed that it is effective to use high-purity metallic silicon having low Al and Ti contents in order to obtain high-purity hexachlorodisilane. If, however, the tendency that chlorination reaction is less likely to occur as the purity of metallic silicon increases is taken into consideration, the chlorination reaction of high-purity metallic silicon has to be conducted at a high temperature. Increased reaction temperature may decrease the durability of a reaction apparatus and increase the cost. Meanwhile, undesired accumulation of metallic copper or a copper compound occurs in the method of adding the metallic copper or copper compound to silicon as mentioned above. No solution to the problem has been provided. That is, the manufacturing of high-purity chloropolysilane has been demanded from industry but the development of a specific industrial manufacturing method has been an unsolved challenge.